The present invention relates generally to spark-ignition engines, and more particularly, to optimizing lean-burn operation with the use of ionization detection in a natural gas lean-burn engine and preventing engine knock by accurately detecting incipient knock and controlling engine operation in response thereto.
Spark-ignition (SI) reciprocating engines typically have a feasible operating regime determined by air-fuel ratio and spark timing. Such engines typically operate with a safety margin of 0.5% O2, or with a 5-6 degree timing margin. This is commonly referred to as knock margin. Conventional spark-ignition engines typically operate near the stoichiometric air/fuel ratio and depend upon exhaust after treatment with catalytic converters to reduce the nitrogen oxide (NOx) emissions. With increased emissions standards in the recent years, the industry is moving toward lean-burn operation despite the difficulty of maintaining a stable combustion process in such engines due to a relatively large coefficient of variation (COV). By running lean (i.e. operating with an air-fuel ratio greater than 1.4), turbocharged engines can enhance fuel efficiency without sacrificing power while producing less NOx pollutants than conventional stoichiometric engine operation. However, such operation is limited by engine knock which typically occurs during lean-burn operation. In order to obtain a maximum power and optimized fuel economy for lean-burn operation, it is desirable to detect the onset of engine knock and to operate near the knock limit (e.g. with reduced knock margin) without damaging the engine.
Accelerometer-based knock sensors are commonly used for detecting knock in SI engines. Accelerometers are mounted to the engine block to detect the high frequency vibrations generated during knocking. However, they are highly susceptible to electrical noise, and knock sensing can be compromised by engine mechanical noises like vibrations during valve closure or piston slapping, especially at high engine speeds. Thus, the signal typically must be filtered, reducing the overall sensitivity of the sensor and hindering such sensors from detecting incipient knock. Incipient knock is defined as a miniscule knock that does not contain a knock frequency that is adverse to engine operation. In essence, sensing incipient knock as an indicator of impending knock production would be useful in controlling engine operation and avoiding knock all together.
In-cylinder pressure sensors have been used to provide direct information about the intensity of knock, which makes them more valuable for knock detection than accelerometers. However, due to the high cost of these sensors and the costs associated with setup and operation thereof, they are used mainly in laboratory settings and are not practical for high-volume field applications.
In-cylinder ion sensors have been used in recent years as a lower cost alternative to the abovementioned knock sensors. They provide a direct measure of in-cylinder thermodynamic conditions and can provide information about knock intensity. However, in lean-burn operation, because of the lean nature of the mixture, the ionized species concentration is much less than at stoichiometric conditions. Thus, integrating the signal cannot be done reliably due to a number of factors that include high levels of noise relative to the ion signal magnitude, variability of the ion signal, and low magnitudes of a resultant integrated signal. An ion sensor in a lean-burn engine also tends to exhibit great variability, typically due to changes in fuel content, temperature, and humidity. However, these systems are also not sensitive enough to detect the onset of incipient knock. For example, a knock detection system employing knock frequency measurement will only detect strong detonation, not incipient knock, as described above, which contains virtually no spectral content.
Thus, the techniques developed using ion sensors for stoichiometric operation are unsuitable for lean-burn operation and previous knock detection systems have been limited thereto.
It would therefore be desirable to have a system and method capable reliably and affordably detecting incipient knock in a SI engine and control operation of the engine to avoid entering into a frequency producing knock condition.